1. Field of the Invention
The present invention generally relates to the synthesis of a photosensitive precursor of pentacene. More particularly, this invention relates to using a pentacene thin film formed from a photothermal conversion of these precursors in the fabrication of organic thin film transistors. This invention also relates to patterning of thin films of these materials by exposure to ultraviolet (UV) light through a mask.
2. Description of the Related Art
Organic semiconductors have received increasing attention during the past two decades due to potential low cost applications in electronic devices (see R. H. Friend et al. Electroluminescence in conjugated polymers, Nature, 1999, 397, 121. and A. J. Heeger, Semiconducting and metallic polymers”, Angew. Chem. Int. Ed., 2001, 40, 2591). The main applications of these compounds are seen in the field of light-emitting diodes (see, Leung, L. M. High efficiency blue emitter for small molecule-based organic light emitting diodes, J. Am. Chem. Soc., 2000, 122, 5640) and large area electronics (see, C. D. Dimitrakopoulos et al., Organic Thin Film Transistors for Large Area Electronics, Adv. Mater., 2002, 14, 99). Both applications require the organic semiconducting material to be patterned.
Unlike inorganic semiconductors, standard wet-process, lithographic techniques can not be used for organic semiconductors due to contamination of these materials by photoresists and/or developers used in patterning of the photoresists. Several techniques have been used for patterning of the organic semiconductors. One of the first approaches used for depositing a pattern with organic semiconductors was screen printing (see Garnier, F. et al., All-polymer field effect transistors, Science, 1994, 265, 1684). This approach is particularly desirable from the viewpoint of device fabrication because it is an additive process, wherein materials are deposited only where required. The drawbacks for screen printing of organic semiconductors are the limited resolution (50–200 micrometers) and the undesirably low solution viscosity of the organic semiconductors.
Another method of patterning organic semiconductors, which has gained considerable attention, is ink-jet printing. Ink-jet printing relies on the steering of material-containing droplets, discharged from a nozzle, and their contained impact and solidification onto a target substrate. This technique has been used in the fabrication of an all-polymer transistor circuits (see, Sirringhaus, H. et al., High resolution ink-jet printing of all-polymer transistor circuits, Science, 2000, 290, 2123) with a maximum resolution of about 25 micrometer.
Finally, photochemical patterning which is the process used in imaging the inorganic semiconductors by using a photoresist material, has been reported for the patterning of π-conjugated polymers. For example, a polythiophene with a pendant tetrahydropyranyl (THP) functionality was patterned using a “chemical amplification” method (see Yu, J. et al., Chemically amplified photolithography of a conjugated polymera, Chem. Commun., 1998, 1503). In this example as shown in Scheme I, thin films of poly(3-(2-(2-tetrahydropyranyloxy)ethyl)thiophene) are deposited together with a photoacid generator. This thin film is thermally stable up to 200° C. However, when acid is generated in the film by exposure to UV light, catalytic cleavage of the tetrahydropyranyl group occurs at 100° C. and the polymer is rendered insoluble.
This approach to patterning, which has been used for substituted polythiophenes and poly(phenylenevinylene), in principle results in high resolution patterning of conjugated polymers but the polymers have very low electrical charge (field-effect) mobility.